1. Field of the Invention
This invention relates to a method for producing a group III nitride semiconductor single crystal and a method for producing a free-standing substrate of a group III nitride semiconductor single crystal.
2. Description of the Related Art
For example, a conventional method for producing a free-standing substrate of a group III nitride semiconductor single crystal is conducted such that a GaN layer is grown on a hetero-substrate such as sapphire by HVPE (halide vapor phase epitaxy) and the hetero-substrate is removed to have a GaN free-standing substrate after the GaN growth is completed (See JP-B-3631724).
In the above method, a base substrate is formed by growing a first group III nitride semiconductor layer on a base material, a metal film is formed on the base substrate, voids are formed in the first group III nitride semiconductor layer through openings in the metal film by heat treatment, and a second group III nitride semiconductor layer is grown on the first group III nitride semiconductor layer with the voids formed therein. By using the method, the second group III nitride semiconductor layer is grown as a single crystal which yields a group III nitride semiconductor substrate with low dislocation density, good crystalline quality and no warping, and allows the easy removal of the hetero-substrate.
On the other hand, another method is proposed that an ingot of GaN single crystal is grown by using a GaN crystal as a seed crystal by HVPE, and plural GaN substrates are cut out from the grown ingot (See JP-A-2003-527296).
However, the method as disclosed in JP-B-3631724 has the drawback that it is not suited to produce the plural group III nitride semiconductor single crystal substrates at once since only one group III nitride semiconductor single crystal substrate can be obtained by each growth of the method. Also, the method as disclosed in JP-A-2003-527296 has the drawback that it only provides for a reduced area in cutting out the substrate due to a difference in crystal face orientation caused when growing the group III nitride semiconductor single crystal on the seed crystal.